This invention generally relates to new developers and the use of such developers for developing images in an electrophotographic system. More specifically, the present invention is concerned with new developers containing compatible quaternary ammonium compounds and the use of such developers for developing electrostatic latent images.
In the electrophotographic process and more specifically the xerographic process, a plate generally comprising a conductive backing upon which is placed a photoconductive insulating surface is uniformly charged, and subsequently the photoconductive surface is exposed to a light image of the original to be reproduced. The photoconductive surface is made in such a manner so as to cause it to become conductive under the influence of the light image in order that the electrostatic charge found thereon can be selectively dissipated to produce what is developed by means of a variety of pigmented resin materials specifically made for this purpose, such as toners. The toner material used is electrostatically attracted to the latent image areas on the plate in proportion to the charge concentration contained thereon. For example, areas of high charge of concentration become areas of high toner density and correspondingly low charge images become proportionately less dense. Subsequently, the developed image is transferred to a final support material such as paper and fixed thereto for a permanent record or copy of the original.
Many methods are known for applying the electroscopic particles to the electrostatic latent image to be developed such as, for example, the development method described in E. N. Wise, U.S. Pat. No. 3,618,552, "Cascade Development". Another method of developing electrostatic latent images is in the magnetic brush process as disclosed for example in U.S. Pat. Nos. 2,874,063; 3,251,706 and 3,357,402. In this method a developer material containing toner and magnetic carrier particles is carried by a magnet with the magnetic field of the magnet causing alignment of the magnetic carrier into a brush like configuration. The magnetic brush is brought in close proximity of the electrostatic latent image bearing surface and the toner particles are drawn from the brush to the electrostatic latent image by electrostatic attraction. Other methods of development include for example powder cloud development as described in C. F. Carlson U.S. Pat. No. 2,221,776, touchdown development as described in R. W. Gundlach U.S. Pat. No. 3,166,432 and cascade development as described in U.S. Pat. No. 3,099,943.
The image itself can be fixed by using a number of different techniques including for example vapor fixing, heat fixing, pressure fixing or combinations thereof as described for example in U.S. Pat. No. 3,539,161.
Many times it is desirable in electrophotographic systems to produce a reverse copy of the original. Thus, for example, it may be desired to produce a negative copy from a positive original or a positive copy from a negative original. Normally this is referred to as image reversal and in electrostatic printing image reversal can be accomplished by applying to the image a developer powder which is repelled by the charged areas of the image and adheres to the discharged areas. Also such reversal developers or more specifically toners containing positive charges are found to be very useful and effective in electrophotographic systems using organic photoreceptors which in many instances are initially charged negatively rather than positively thereby necessitating the need for a positively charged toner. Reversal developers are described in U.S. Pat. No. 2,986,521, such developers being comprised of electroscopic material coated with finely divided colloidal silica. When used in an electrostatic development system, development of electrostatic images on negatively charged surfaces is accomplished by applying the electroscopic material having a positive triboelectric relationship with respect to the colloidal silica.
These developers have a tendency to lose their positive charge over a period of time, are difficult to prepare and as a result the quality of the image can be adversely affected over a period of time. The use of charge control agents in developers is known, for example as described in U.S. Pat. No. 3,893,935, but such materials are soluble in water, causing them to be leached to the toner surface and also toners containing these materials are humidity sensitive. Also, these materials are incompatible with thermoplastic resins and it is very difficult to uniformly disperse or dissolve such materials in the toner. This causes particle-to-particle non-uniformity and wide distribution of electrical charge which reduces the quality of the image developed.
Accordingly, there is a need for a developer which can be used in a reversal system, and more specifically there is a need for a positively charged toner that has improved resistance to moisture leaching, is of improved pigment dispersion and of improved toner particle-to-particle uniformity.